1. Field of the Invention
The present invention is directed to an injection reservoir which is self-sealing to punctures by a hypodermic needle. More particularly, the present invention is directed to a self-sealing injection reservoir of the type which is implanted under the skin of humans and domestic animals in connection with tissue expanders and various drug delivery systems.
2. Brief Description of the Prior Art
The prior art is well aware of tissue expanding devices which are temporarily implanted under the skin of humans and domestic animals, and which are gradually inflated by injection of saline or similar liquids for the purpose of causing the growth of a skin flap or enlargement. For a detailed description of such tissue expanders, reference is made to U.S. Pat. No. 4,217,889. Briefly summarized, such tissue expanders comprise a surgically implantable bag into which the liquid is introduced to gradually enlarge the bag and thereby to cause the desired skin enlargement or flap formation. After the skin flap has formed, the bag is surgically removed. The skin flap is either used, after severance, for purposes of plastic surgery in other parts of the body, or accommodates a permanent implantation directly beneath the flap. Neither the manner of using the skin flap, nor the permanent implants, form part of the present invention.
For medically self-evident reasons the temporarily implantable tissue expander bag must not leak, and must not be punctured to form a leak, when, from time to time, additional liquid is introduced into the bag. In some situations, liquid must even be withdrawn from the bag without causing a leak. Such addition or withdrawal of liquid is accomplished, in accordance with standard practice in the art, through a substantially non-expandable bioimplantable injection button or reservoir, which is self-sealing to punctures by a hypodermic needle. The self-sealing, non-expandable injection button or reservoir is usually connected to the bag, through a suitable bioimplantable duct or narrow tubing. In some tissue expanders, the injection reservoir is incorporated in the wall of the bag. In other words, to gradually inflate the tissue expander bag liquid is introduced into the bag through a distinct, non-expandable but self-sealing injection reservoir, which is temporarily implanted beneath the skin together with the tissue expander bag.
Injection buttons or reservoirs of the prior art are described in U.S. Pat. Nos. 4,190,040 and 4,428,364.
More particularly, the injection reservoir of U.S. Pat. No. 4,190,040 includes a substantially flat base and a generally dome-shaped, double-walled upper portion, which, together with the base, forms an enclosure or reservoir chamber for the injected liquid. The enclosure is connected to the tissue expander bag with a suitable tubing or duct. The self-sealing capability of the double-walled upper portion is attained by a silicone gel sealant layer which is contained between the two walls. In order to protect the base from puncture by accidentally overextending the hypoderminc needle into the base, a non-puncturable plate member is contained within the enclosure to cover the base.
The injection reservoir of U.S. Pat. No. 4,428,364, on the other hand, is contained within the tissue expander bag itself. In this device, a generally dome-shaped wall of the reservoir has multiple layers of fabric reinforced silicone rubber sheets, with the weave of the fabric in the several layers being oriented in multiple directions. A swelling agent is present in the dome-shaped wall. The forces created by the swelling of the silicone rubber sheets and the restraining force of the fabric cause a compressive stress in the wall, which, in turn, renders the wall self-sealing to punctures by a hypodermic needle.
Although the above-described self-sealing injection buttons or reservoirs function, by-and-large, adequately, improvements are needed, particularly from the standpoint of ease and cost of manufacturing and resistance to leakage. The present invention provides such an improvement.